А что если перестроить сохранения повтора для?
In dreaming there's no harm - the harm is in doing nothing.

Elena, you're two sentences are both correct, but they don't express the contrast inherent in this statement. If you worded it as one sentence and stressed one situation over the other, it would be more idiomatic: "Dreaming isn't harmful, it's idleness that's harmful."

Roman, the problem with your suggestion is that we don't say "In dreaming there's no harm" in this context. It's a strange syntax.

Thanks for the hint, Uly 🙋

Absolutely

Thanks, Uly) There is an emphasis on "вредно" in the original, that's why I tried inversion